1. Field of the Invention
This invention relates to the field of converting digital data representing figures or objects into a pixel format for printing on a pattern generation system.
2. Prior Art
In the photolithographic fabrication of integrated circuits, resist sensitive to radiant particle energy is exposed in predetermined patterns to define circuit features. In some cases, the energy is passed through masks which contains the patterns, thereby selectively exposing a photoresist on a semiconductor body. In other instances, the resist is on a mask substrate and the direction of the radiant energy itself is controlled to define patterns in the resist. The selective exposure can be done as part of the making a mask (or reticle) or to directly "write" onto the resist film covering a semiconductor wafer. Several sources of radiant energy have been used, including ultraviolet light, visible light, coherent light, x-rays and electron beam (E-Beam).
A system for photolithographic fabrication of integrated circuits is described in U.S. Pat. No. 4,796,038 entitled "Laser Pattern Generation Apparatus" which is assigned to the assignee of the present invention. In U.S. Pat. No. 4,796,038 circuit patterns are written onto a workpiece by directing laser beams and moving a workpiece relative to the laser beams. A single laser beam is split into eight (8) beams to form a brush. The beams pass through Acousto-Optical Modulator (AOMs). The AOMs receive electric signals defining the circuit patterns. The AOMs are used to control the intensity of the beams as they write onto the workpiece. A steering mirror is used for directing the plurality of beams through a zoom lens arrangement and towards a rotating polygonal mirror. The steering mirror is used to adjust and align the beams in response to movement of the workpiece. The zoom lens arrangement is for adjusting the size and placement of the beams. The rotating polygonal mirror has a plurality of facets and is used to scan the beams onto the workpiece. Through successive scans, a stripe (one pass of the stage) is printed on the workpiece. The stripes comprise the different portions of the integrated circuit pattern. All stripes of a circuit are then printed on a workpiece. Thus, the circuit pattern is written onto the workpiece.
Commercially available systems embodying the laser pattern generation apparatus of U.S. Pat. No. 4,796,038 include the CORE 2100, 2500 and WAFER WRITE-6000 Systems available from Etec Systems, Inc. of Beaverton, Oreg.
As noted above, the electric signals which are received by the AOM's are generated by a rasterizer. By rasterized it is meant to convert the pattern into a collection of pixel values. The pixel values represent the electrical signals which are sent to the AOMs. Such a rasterizer is described in U.S. Pat. No 4,806,921, entitled "Rasterizer for Pattern Generator" which is assigned to the assignee of the present invention.
A method for improved control of modulation of radiant energy beams is found in U.S. Pat. No. 4,956,650 entitled "Pattern Generation System" which is assigned to the assignee of the present invention. This allows for correction of non-linearities in the turn on/turn off of the beams.
Rasterization is conceptually similar to creating a bit-mapped image of the circuit pattern. The bit-mapped image contains a plurality of pixel values. Each pixel value corresponds to an electrical signal sent to the AOMs. In order to obtain the fine detail needed for printing integrated circuits, the address grid is finer than the pixel grid.
The flow for providing the electrical signals to the AOMs is described with reference to FIG. 1. Referring to FIG. 1, a file 101 containing geometric instructions and placement instructions is provided to rasterizer 102. The file 101 may be in one of many commercially known writing formats, e.g. the MEBES.RTM. format or the ALF format. The rasterizer 102 creates a bit-mapped image 103 of the circuit pattern. Thus, the bit-mapped image of the pattern is printed many times on the wafer. The bit-mapped image 103 is then provided to the AOMs 104. The AOMs 104 control the exposure of the laser beams onto the workpiece, which effectively controls the printing operation.
Known rasterizers, are limited in their rendering speed, 2 bit intensities and their flexibility. Thus, it is an object of the present invention to provide a rasterizer which can supply multiple pixel intensities at an increased speed and which will allow scalability.